Pilot symbol transmission in wireless communication systems

ABSTRACT

Pilot symbols transmitted from different sectors of a same base station are multiplied with a same cell specific scrambling code and a first code having low cross correlation and second codes having low cross correlation. The second code is constant over the length of the first code, but may vary for repetitions of the first code.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent claims priority to ProvisionalApplication No. 60/731,111 entitled “PILOT SYMBOL TRANSMISSION INWIRELESS COMMUNICATION SYSTEMS” filed Oct. 27, 2005, and assigned to theassignee hereof and hereby expressly incorporated by reference herein.

BACKGROUND

I. Field

The present document relates generally to wireless communication and,amongst other things, to pilot transmission in wireless communication.

II. Background

A wireless communication system may utilize multiple transmit antennasat a base station or user station to transmit symbols or otherinformation. The use of multiple transmit antennas improves the receiverability to decode symbols since multiple versions of a same symbol areavailable to use in decoding the transmissions.

An orthogonal frequency division multiple access (OFDMA) system utilizesorthogonal frequency division multiplexing (OFDM). OFDM is amulti-carrier modulation technique that partitions the overall systembandwidth into multiple (N) orthogonal frequency subcarriers. Thesesubcarriers may also be called tones, bins, and frequency channels. Eachsubcarrier is associated with a respective sub carrier that may bemodulated with data. Up to N modulation symbols may be sent on the Ntotal subcarriers in each OFDM symbol period. These modulation symbolsare converted to the time-domain with an N-point inverse fast Fouriertransform (IFFT) to generate a transformed symbol that contains Ntime-domain chips or samples.

In a frequency hopping communication system, data is transmitted ondifferent frequency subcarriers in different time intervals, which maybe referred to as “hop periods”. These frequency subcarriers may beprovided by orthogonal frequency division multiplexing, othermulti-carrier modulation techniques, or some other constructs. Withfrequency hopping, the data transmission hops from subcarrier tosubcarrier in a pseudo-random manner. This hopping provides frequencydiversity and allows the data transmission to better withstanddeleterious path effects such as narrow-band interference, jamming,fading, and so on.

An OFDMA system can support multiple mobile stations simultaneously. Fora frequency hopping OFDMA system, a data transmission for a given mobilestation may be sent on a “traffic” channel that is associated with aspecific frequency hopping (FH) sequence. This FH sequence indicates thespecific subcarrier to use for the data transmission in each hop period.Multiple data transmissions for multiple mobile stations may be sentsimultaneously on multiple traffic channels that are associated withdifferent FH sequences. These FH sequences may be defined to beorthogonal to one another so that only one traffic channel, and thusonly one data transmission, uses each subcarrier in each hop period. Byusing orthogonal FH sequences, the multiple data transmissions generallydo not interfere with one another while enjoying the benefits offrequency diversity.

An accurate estimate of a wireless channel between a transmitter and areceiver is normally needed in order to recover data sent via thewireless channel. Channel estimation is typically performed by sending apilot from the transmitter and measuring the pilot at the receiver. Thepilot signal is made up of pilot symbols that are known a priori by boththe transmitter and receiver. The receiver can thus estimate the channelresponse based on the received symbols and the known symbols.

A code division multiple access (CDMA) system has a universal frequencyreuse that makes it possible for mobile users to receive and send thesame signal simultaneously from and to multiple base stations or sectorsof a base station. Soft and softer handoff in CDMA systems aretechniques whereby mobiles near cell, and sector in the case of softerhandoff, boundaries communicate the same transmitted signals to morethan one base station or sector of a base station. Soft and softerhandoff provides enhanced communication quality and a smoothertransition compared to the conventional hard handoff. Soft and softerhandoff is intrinsic to a CDMA system, as transmitted signals ofdifferent users occupy the same time and frequency allocation. Differentusers can be separated based on the respective spreading signatures.

Supporting soft and softer handoff in orthogonal multiple-access systemssuch as TDMA, FDMA and OFDMA is far more difficult and often requiresspecial planning. For instance, in order to provide diversity acell-specific scrambling code is used in the forward link to randomizethe interference from the surrounding cells. Often, the scrambling codeis different among sectors in the same cell (i.e., Node B). When thesector-specific scrambling code is applied to OFDM-based radio access inthe downlink, each modulation symbol of the pilot channel suffers frominter-sector interference. The influence of inter-sector interference onthe pilot channel is significant particularly for a user in handoff.However, the channel estimation using the pilot channel in theinter-sector diversity is not improved compared to the case with aone-link connection due to the inter-sector interference. Therefore,improvement in the channel estimation especially in inter-sectorhandover is essential.

Therefore, there is a need to find efficient approaches to provideimproved channel estimation for different sectors in OFDMA systems.

SUMMARY

In certain aspects, a wireless communication apparatus comprises amemory and a circuit coupled with the processor. The memory isconfigured to store at least one first sequence of a group of firstsequences, having a low cross-correlation with each other, of a firstlength and at least two second sequences of a group of second sequences,having a low cross-correlation with each other, of a second length thatis different than the first length. The circuit is configured tomultiply pilot symbols to be transmitted from a plurality of antennagroups utilizing the at least one first sequence and the at least twosecond sequences.

In another aspect, a method of transmitting pilot symbols comprisesmultiplying first pilot symbols for a first antenna group utilizing afirst sequence of a group of first sequences, having a lowcross-correlation with each other, and at least two second sequences ofa group of second sequences, having a low cross-correlation with eachother, and multiplying second pilot symbols for a second antenna grouputilizing a third sequence of the group of first sequences and at leasttwo fourth sequences of the group of second sequences. The first andsecond pilot symbols may be transmitted.

In further aspects, a processor readable medium may include instructionsexecutable by one or more processors. The instructions may perform oneor more aspects of the above method.

In an additional aspect, an apparatus for transmitting pilot symbolscomprises means for multiplying first pilot symbols for a first antennagroup utilizing a first sequence of a group of first sequences, having alow cross-correlation with each other, and at least two second sequencesof a group of second sequences, having a low cross-correlation with eachother, and means for multiplying second pilot symbols for a secondantenna group utilizing a third sequence of the group of first sequencesand at least two fourth sequences of the group of second sequences. Theapparatus may also include one or more transmitters configured totransmit the first and second pilot symbols.

Various aspects and embodiments are described in further detail below.The applications further provide methods, processors, transmitter units,receiver units, base stations, terminals, systems, and other apparatusesand elements that implement various aspects, embodiments, and features,as described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present embodiments maybecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates a multiple access wireless communication systemaccording to one embodiment;

FIG. 2 illustrates a spectrum allocation scheme for a multiple accesswireless communication system according to one embodiment;

FIG. 3 illustrates aspects of a multiple access wireless communicationsystem;

FIG. 4 illustrates a block diagram of aspects of a transmitter systemand a receiver system in a multi-input multi-output multiple accesswireless communication system;

FIG. 5 illustrates a flow chart of aspects of a method of pilottransmission; and

FIG. 6 illustrates a block diagram of aspects of portions of amulti-sector base station.

DETAILED DESCRIPTION

Referring to FIG. 1, a multiple access wireless communication systemaccording to one embodiment is illustrated. A base station 100 includesmultiple antenna groups 102, 104, and 106 each including one or moreantennas. In FIG. 1, only one antenna is shown for each antenna group102, 104, and 106, however, one or multiple antennas may be utilized foreach antenna group that corresponds to a sector of base station 100.Mobile station 108 is in communication with antenna 104, where antenna104 transmits information to mobile station 108 over forward link 118and receives information from mobile station 108 over reverse link 116.Mobile station 110 is in communication with antenna 106, where antenna106 transmits information to mobile station 110 over forward link 126and receives information from mobile station 110 over reverse link 128.

Each group of antennas 102, 104, and 106 and/or the area in which theyare designed to communicate is often referred to as a sector of the basestation. In the embodiment, antenna groups 102, 104, and 106 each aredesigned to communicate to mobile stations in a sector, sectors 120,122, and 124, respectively, of the areas covered by base station 100.

In order to allow for efficient processing of data symbols, base station100 may transmit pilot symbols from sectors 120, 122, and 124 that maybe identified as being different from each other. In certain aspects,this may be performed even if the pilots are transmitted on the samesubcarriers and at approximately the same time from two sectors at onetime. For example, this may be performed by multiplying the pilotsymbols of each sector by a common scrambling code for the cell and afirst code with low cross-correlation, e.g. an orthogonal code, which isspecific for the particular sector. In this way, interference for pilotsymbols transmitted from a given sector is decreased with respect to anyother sector. In addition to the first code for the sector, a secondcode with low cross-correlation, e.g. an orthogonal code,quasi-orthogonal, or a PN code, may be used for each portion of the codethat is repeated in a given OFDM symbol. Also, the second code with lowcross-correlation are selected so that the pilot symbols transmittedfrom a given sector code with low cross-correlation with respect to thepilot symbols transmitted from another sector of a same or differentcell.

A base station may be a fixed station used for communicating with theterminals and may also be referred to as, and include some or all thefunctionality of, an access point, a Node B, or some other terminology.A mobile station may also be referred to as, and include some or all thefunctionality of, a mobile station, a user equipment (UE), a wirelesscommunication device, terminal, access terminal or some otherterminology.

As used herein, in communication with antenna group or antenna generallyrefers to the antennas group or antenna that is responsible fortransmission to a mobile station. In the case of transmission from amobile station, multiple antenna groups may be utilized to receivetransmissions including utilizing soft or other types of combining.

It should be noted that while FIG. 1, depicts physical sectors, i.e.having different antenna groups for different sectors, other approachesmay be utilized. For example, utilizing multiple fixed “beams” that eachcover different areas of the cell in frequency space may be utilized inlieu of, or in combination with physical sectors. Such an approach isdepicted and disclosed in copending U.S. patent application Ser. No.11/260,895, entitled “Adaptive Sectorization In Cellular System,” andfiled on Oct. 27, 2005, and which is incorporate herein by reference inits entirety. In such a case, different “beams” may be assigneddifferent scrambling sequences and code with low cross-correlation asdescribed herein.

As discussed above, a first code with low cross-correlation is used tomultiply the pilot symbols at each sector that is orthogonal orquasi-orthogonal to another first code with low cross-correlation usedto multiply the pilot symbols of each other sector of the cell. Inaddition, a second code with low cross-correlation is utilized tomultiply the pilot symbols after multiplication by the first code ineach sector. This second code with low cross-correlation, which may beany number of orthogonal or quasi-orthogonal codes, is used to multiplyover the length of the first code, with a different second code with lowcross-correlation used in a single sector. This is illustrated in Table1 below:

TABLE 1 Sector 1 Sector 2 Sector 3 Pilot Pilot Pilot Sub- First SecondSub- First Second Sub- First Second carrier Code Code carrier Code Codecarrier Code Code f₀ W₁(0) P₁(0) f₀ W₂(0) P₃(0) f₀ W₃(0) P₅(0) f₂ W₁(2)P₁(2) f₂ W₂(2) P₃(2) f₂ W₃(2) P₅(2) f₄ W₁(4) P₁(4) f₄ W₂(4) P₃(4) f₄W₃(4) P₅(4) f₆ W₁(6) P₁(6) f₆ W₂(6) P₃(6) f₆ W₃(6) P₅(6) f₈ W₁(8) P₂(8)f₈ W₂(8) P₄(8) f₈ W₃(8) P₆(8)  f₁₀  W₁(10)  P₂(10)  f₁₀  W₂(10)  P₄(10) f₁₀  W₃(10)  P₆(10)  f₁₂  W₁(12)  P₂(12)  f₁₂  W₂(12)  P₄(12)  f₁₂ W₃(12)  P₆(12)  f₁₄  W₁(14)  P₂(14)  f₁₄  W₂(14)  P₄(14)  f₁₄  W₃(14) P₆(14)

In the examples of Table 1, pilots occupy tones f₀, f₂, f₄, f₆, f₈, f₁₀,f₁₂, and f₁₄ in a given OFDM symbol for all sectors of a given cell. Acell specific scrambling code may or may not be applied. In some aspectsthis cell specific scrambling code may be applied as S(0), S(2), S(4),S(6), S(8), S(10), S(12), and S(14). This is then multiplied by a firstcode W₁(0), W₁(2), W₁(4), W₁(6), W₁(8), W₁(10), W₁(12), and W₁(14) forsector 1, a first code W₂(0), W₂(2), W₂(4), W₂(6), W₂(8), W₂(10),W₂(12), and W₂(14) for sector 2, and a first code W₃(0), W₃(2), W₃(4),W₃(6), W₃(8), W₃(10), W₃(12), and W₃(14) for sector 3. In some aspects,the first and, or, second codes W₁, W₂, and W₃ may be Walsh orExponential codes. Also, each of the first code generally has a lengththat is less than a number of tones utilized for pilot symbols. Forexample, the codes W₁, W₂, and W₃ may have a length four, such that eachfirst code is repeated twice in each OFDM symbol for each sector. Thatis, the first code used to multiply the pilot symbols for subcarriersf₀, f₂, f₄, and f₆, and the first code used to multiply the pilotsymbols for subcarriers f₈, f₁₀, f₁₂, and f₁₄ is identical. The receivedsignal at tone i for the user is given by S(i)(W₁(i)P_(N)(i)H₁(i)+W₂(i)P_(N)(i)H₂(i)+W₃(i)P_(N)(i)H₃(i))+noise, whereH₁, H₂ and H₃ are the channels from sectors 1, 2, and 3 respectively.

Even though W₁, W₂, and W₃ may be orthogonal or quasi-orthogonal to eachother over tones {0, 2, 4, 6}, the products of any two of W₁(i)H₁(i),W₂(i)H₂(i), and W₃(i)H₃(i) may not be orthogonal or quasi-orthogonal, ifany channel H₁, H₂, or H₃ shows significant variations over the tone set{0, 2, 4, 6, 8, 10, etc.} In some cases, because of the channelvariation, any two of W₁(i)H₁(i), W₂(i)H₂(i), and W₃(i)H₃(i) actuallyprovide a very high correlation. In this case, there will be nosuppression of the interference from pilots of the two sectors. Toaddress such situations the second code P_(N) may be constant over thelength of the first code. In some aspects, the second code would takeone pseudorandom value for tones f₀, f₂, f₄, and f₆, and another valuefor tones f₈, f₁₀, f₁₂, and f₁₄. This provides that even if W₁(i)H₁(i),W₂(i)H₂(i), and W₃(i)H₃(i) from two sectors have a high correlation,this correlation has a different phase for different regions offrequency due to the code P_(N) used, and across different timeinstances. When an interpolation is performed across frequency or time,the different phases will average out to yield a low overallcorrelation.

While the second codes may be constant over the length of the firstcode, they may vary for other subcarriers that extend beyond the lengthof the first code in a same sector. It should be noted that the secondcodes may be different for each sector. Alternatively, the second codesmay be the same in each sector, but have a different order in eachsector as applied to the different pilot subcarriers in order to makesure no pilot symbols are multiplied by a same second code.Additionally, the sectors may utilize different combinations of some ofthe same or some of the different codes.

In some cases, it may be desired that the pilots will have to occupy thesame set of tones in different sectors. Therefore, if pilot hopping isused, we have to ensure that it is the same for different sectors.

Referring to FIG. 3, a multiple access wireless communication systemaccording to another embodiment is illustrated. A multiple accesswireless communication system 600 includes multiple cells, e.g. cells602, 604, and 606. In the embodiment of FIG. 3, each cell 602, 604, and606 may include multiple sectors, not shown, which are in communicationwith mobile stations 620. As discussed above, each cell 602, 604, and606 may utilize a different cell specific scrambling code to multiplypilot symbols transmitted from its sectors. Each sector may then operateaccording to any of the aspects and utilize any of the featuresdescribed herein.

Referring to FIG. 4, a block diagram of an embodiment of a transmittersystem 810 and a receiver system 850 in a MIMO system 800 isillustrated. At transmitter system 810, traffic data for a number ofdata streams is provided from a data source 812 to transmit (TX) dataprocessor 814. In an embodiment, each data stream is transmitted over arespective transmit antenna. TX data processor 814 formats, codes, andinterleaves the traffic data for each data stream based on a particularcoding scheme selected for that data stream to provide coded data.

The coded data for each data stream may be multiplexed with pilot datausing OFDM techniques. The pilot data is typically a known data patternthat is processed in a known manner and may be used at the receiversystem to estimate the channel response. The multiplexed pilot and codeddata for each data stream is then modulated (i.e., symbol mapped) basedon a particular modulation scheme (e.g., BPSK, QSPK, M-PSK, or M-QAM)selected for that data stream to provide modulation symbols. The datarate, coding, and modulation for each data stream may be determined byinstructions performed on provided by processor 830.

The modulation symbols for all data streams are then provided to a TXprocessor 820, which may further process the modulation symbols (e.g.,for OFDM). TX processor 820 then provides N_(T) modulation symbolstreams to N_(T) transmitters (TMTR) 822 a through 822 t. Eachtransmitter 822 receives and processes a respective symbol stream toprovide one or more analog signals, and further conditions (e.g.,amplifies, filters, and upconverts) the analog signals to provide amodulated signal suitable for transmission over the MIMO channel. N_(T)modulated signals from transmitters 822 a through 822 t are thentransmitted from N_(T) antennas 824 a through 824 t, respectively.

At receiver system 850, the transmitted modulated signals are receivedby N_(R) antennas 852 a through 852 r and the received signal from eachantenna 852 is provided to a respective receiver (RCVR) 854. Eachreceiver 854 conditions (e.g., filters, amplifies, and downconverts) arespective received signal, digitizes the conditioned signal to providesamples, and further processes the samples to provide a corresponding“received” symbol stream.

An RX data processor 860 then receives and processes the NR receivedsymbol streams from NR receivers 854 based on a particular receiverprocessing technique to provide NT “detected” symbol streams. Theprocessing by RX data processor 860 is described in further detailbelow. Each detected symbol stream includes symbols that are estimatesof the modulation symbols transmitted for the corresponding data stream.RX data processor 860 then demodulates, deinterleaves, and decodes eachdetected symbol stream to recover the traffic data for the data stream.The processing by RX data processor 860 is complementary to thatperformed by TX processor 820 and TX data processor 814 at transmittersystem 810.

RX processor 860 may derive an estimate of the channel response betweenthe N_(T) transmit and N_(R) receive antennas, e.g., based on the pilotinformation multiplexed with the traffic data. RX processor 860 mayidentify the pilot symbols according to pilot patterns stored in memory,e.g. memory 872 that identify the frequency subcarrier and symbol periodassigned to each pilot symbol. In addition, the cell scrambling code andsector specific codes, e.g. the second codes with low cross-correlation,may be stored in memory so that they may be utilized by RX processor 860to multiple the received symbols so that the proper decoding can occur.

The channel response estimate generated by RX processor 860 may be usedto perform space, space/time processing at the receiver, adjust powerlevels, change modulation rates or schemes, or other actions. RXprocessor 860 may further estimate the signal-to-noise-and-interferenceratios (SNRs) of the detected symbol streams, and possibly other channelcharacteristics, and provides these quantities to a processor 870. RXdata processor 860 or processor 870 may further derive an estimate ofthe “operating” SNR for the system. Processor 870 then provides channelstate information (CSI), which may comprise various types of informationregarding the communication link and/or the received data stream. Forexample, the CSI may comprise only the operating SNR. The CSI is thenprocessed by a TX data processor 878, modulated by a modulator 880,conditioned by transmitters 854 a through 854 r, and transmitted back totransmitter system 810.

At transmitter system 810, the modulated signals from receiver system850 are received by antennas 824, conditioned by receivers 822,demodulated by a demodulator 840, and processed by a RX data processor842 to recover the CSI reported by the receiver system. The reported CSIis then provided to processor 830 and used to (1) determine the datarates and coding and modulation schemes to be used for the data streamsand (2) generate various controls for TX data processor 814 and TXprocessor 820. Alternatively, the CSI may be utilized by processor 870to determine modulation schemes and/or coding rates for transmission,along with other information. This may then be provided to thetransmitter which uses this information, which may be quantized, toprovide later transmissions to the receiver.

Processors 830 and 870 direct the operation at the transmitter andreceiver systems, respectively. Memories 832 and 872 provide storage forprogram codes and data used by processors 830 and 870, respectively. Thememories 832 and 872 store the cell specific scrambling sequence and thefirst and second codes with low cross-correlation s.

Processors 830 and 870 then can utilize the cell specific scramblingsequence and the first and second codes code with low cross-correlationto multiply the pilot symbols for each sector, as appropriate.

At the receiver, various processing techniques may be used to processthe N_(R) received signals to detect the N_(T) transmitted symbolstreams. These receiver processing techniques may be grouped into twoprimary categories (i) spatial and space-time receiver processingtechniques (which are also referred to as equalization techniques); and(ii) “successive nulling/equalization and interference cancellation”receiver processing technique (which is also referred to as “successiveinterference cancellation” or “successive cancellation” receiverprocessing technique).

While FIG. 4 discusses a MIMO system, the same system may be applied toa multi-input single-output system where multiple transmit antennas,e.g. those on a base station, transmit one or more symbol streams to asingle antenna device, e.g. a mobile station. Also, a single output tosingle input antenna system may be utilized in the same manner asdescribed with respect to FIG. 4.

Referring to FIG. 5, a flow chart of a method of pilot symbol assignmentaccording to one embodiment is illustrated. A plurality of pilot symbolsare generated, block 900. The first code for the sector is then utilizedto multiply the samples of the pilot symbols, block 902. A second codefor the sector is then applied to the pilot symbols, block 904. In somecases, the second code is constant over the length of the first code. Inother cases the second code can be any length and vary over more less ofthe length of the first code. Further, multiple instances of the firstcode may be multiplied by different second codes. The multiplied symbolsare then transmitted, block 906.

Referring to FIG. 6, a block diagram of aspects of portions of amulti-sector base station is illustrated. A base station may includemultiple sectors, here for the purposes of illustration only being two.However, any number of sectors may be utilized, e.g. three as depictedin FIG. 1. Each sector includes a means for multiplying pilot symbolswith a first code and a second codes, blocks 1000 and 1004 respectively.The codes are different from each other and may both be of a samelength, with different second codes being used for each group of pilotsymbols multiplied by the first code. A pair of respective transmittersis utilized to transmit the multiplied pilot symbols, blocks 1002 and1006 respectively.

The techniques described herein may be implemented by various means. Forexample, these techniques may be implemented in hardware, software, or acombination thereof. For a hardware implementation, the processing unitswithin a base station or a mobile station may be implemented within oneor more application specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof.

For a software implementation, the techniques described herein may beimplemented with instruction (e.g., procedures, functions, and so on)that may be utilized by one or more processors to perform the functionsdescribed herein. The instructions may be stored in memory units andexecuted by processors. The memory unit may be implemented within theprocessor or external to the processor, in which case it can becommunicatively coupled to the processor via various means as is knownin the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments may be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A wireless communication apparatus, comprising:at least one processor configured to multiply first pilot symbols for afirst antenna group of a base station utilizing a first code sequence ofa first group of code sequences, having a low cross-correlation witheach other, to further multiply the first pilot symbols utilizing asecond code sequence of a second group of code sequences, having a lowcross-correlation with each other, to map the first pilot symbols to aset of subcarriers determined based on a predetermined pattern, and tosend the first pilot symbols from the first antenna group of the basestation.
 2. The wireless communication apparatus of claim 1, wherein theat least one processor is configured to multiply the first pilot symbolswith a scrambling sequence common to all antenna groups of the basestation.
 3. The wireless communication apparatus of claim 1, wherein theat least one processor is configured to multiply second pilot symbolsfor a second antenna group of the base station utilizing another firstcode sequence of the first group of code sequences, to further multiplythe second pilot symbols utilizing another second code sequence of thesecond group of code sequences, and to send the second pilot symbolsfrom the second antenna group.
 4. The wireless communication apparatusof claim 1, wherein the predetermined pattern is a same pattern for thefirst antenna group and at least one other antenna group of the basestation.
 5. The wireless communication apparatus of claim 1, wherein theat least one processor is configured to vary the predetermined patternover time.
 6. The wireless communication apparatus of claim 1, whereinthe first group of code sequences comprises Walsh codes.
 7. The wirelesscommunication apparatus of claim 1, wherein the first group of codesequences comprises orthogonal codes.
 8. The wireless communicationapparatus of claim 1, wherein the second group of code sequencescomprises pseudo-random number (PN) sequences.
 9. The wirelesscommunication apparatus of claim 1, wherein the second group of codesequences comprises exponential codes.
 10. A method of transmittingpilot symbols comprising: multiplying first pilot symbols for a firstantenna group of a base station utilizing a first code sequence of afirst group of code sequences, having a low cross-correlation with eachother, and further multiplying the first pilot symbols utilizing asecond code sequence of a second group of code sequences, having a lowcross-correlation with each other; mapping the first pilot symbols to aset of subcarriers determined based on a predetermined pattern; andtransmitting the first pilot symbols from the first antenna group. 11.The method of claim 10, further comprising: multiplying the first pilotsymbols by a scrambling sequence common to all antenna groups of thebase station.
 12. The method of claim 10, wherein the predeterminedpattern is a same pattern for the first antenna group and at least oneother antenna group of the base station.
 13. The method of claim 10,further comprising varying the predetermined pattern over time.
 14. Themethod of claim 10, wherein the first group of code sequences comprisesWalsh codes.
 15. The method of claim 10, wherein the first group of codesequences comprises orthogonal codes.
 16. The method of claim 10,wherein the second group of code sequences comprises pseudo-randomnumber (PN) sequences.
 17. The method of claim 10, wherein the secondgroup of code sequences comprises exponential codes.
 18. A wirelesscommunication apparatus comprising: means for multiplying first pilotsymbols for a first antenna group of a base station utilizing a firstcode sequence of a first group of code sequences, having a lowcross-correlation with each other, and for further multiplying the firstpilot symbols utilizing a second code sequence of a second group of codesequences, having a low cross-correlation with each other; means formapping the first pilot symbols to a set of subcarriers determined basedon a predetermined pattern; and a first transmitter configured totransmit the first pilot symbols from the first antenna group.
 19. Theapparatus of claim 18, further comprising means for varying thepredetermined pattern over time.
 20. The apparatus of claim 18, whereinthe first group of code sequences comprises Walsh codes.
 21. Theapparatus of claim 18, wherein the first group of code sequencescomprises orthogonal codes.
 22. The apparatus of claim 18, wherein thesecond group of code sequences comprises pseudo-random number (PN)sequences.
 23. The apparatus of claim 18, wherein the second group ofcode sequences comprises exponential codes.
 24. An article ofmanufacture, comprising a non-transitory machine-readable medium havinginstructions therein that, when read by a machine, configure the machineto: multiply first pilot symbols for a first antenna group of a basestation utilizing a first code sequence of a first group of codesequences, having a low cross-correlation with each other, furthermultiply the first pilot symbols utilizing a second code sequence of asecond group of code sequences, having a low cross-correlation with eachother; map the first pilot symbols to a set of subcarriers determinedbased on a predetermined pattern; and send the first pilot symbols fromthe first antenna group.
 25. The method of claim 10, further comprising:multiplying second pilot symbols for a second antenna group of the basestation utilizing another first code sequence of the first group of codesequences and further multiplying the second pilot symbols utilizinganother second code sequence of the second group of code sequences; andtransmitting the second pilot symbols from the second antenna group. 26.The method of claim 10, wherein the second code sequence is longer thanthe first code sequence.
 27. The method of claim 10, wherein the secondcode sequence has a constant value over the length of the first codesequence.
 28. The method of claim 10, wherein the first code sequenceand the second code sequence are applied across a plurality ofsubcarriers used for pilot transmission.
 29. The method of claim 28,wherein the first code sequence is repeated multiple times across theplurality of subcarriers used for pilot transmission.